1. Field of the Invention
The present invention relates to nickel electrodes for alkaline secondary battery wherein a porous sintered nickel substrate is loaded with a nickel hydroxide-based active material, and alkaline secondary batteries employing the same as the positive electrode therefor. The invention is directed to the improvement of the nickel electrode for alkaline secondary battery for suppression of self discharge associated with oxygen evolution during storage of the charged alkaline secondary battery under high temperature conditions and for increase in the high-current discharge capacity of the electrode.
2. Description of the Related Art
The conventional alkaline secondary batteries such as nickel-hydrogen secondary batteries, nickel-cadmium secondary batteries and the like, have employed sintered nickel electrodes or non-sintered nickel electrodes as the positive electrode therefor.
The non-sintered nickel electrode is fabricated by directly loading a nickel hydroxide-based active material paste into a porous conductive body, such as a nickel substrate foam. Although this electrode is easy to fabricate, there is a disadvantage of poor charge/discharge characteristics at high current.
On the other hand, the sintered nickel electrode employs a porous sintered nickel substrate obtained by sintering and is fabricated by chemically impregnating the porous sintered nickel substrate with a salt as the active material. The sintered nickel substrate presents higher conductivity. In addition, the electrode is excellent in the charge/discharge characteristics at high current because of good adhesion of the active material to the porous sintered nickel substrate. On this account, the alkaline secondary batteries with the sintered nickel electrodes are favorably used in electric power tools requiring high current discharge.
Unfortunately, the sintered nickel electrode has a lower loading ratio of the active material than the non-sintered nickel electrode and therefore, must be improved in the utilization of the active material therefor. In addition, repeated charges/discharges of the alkaline secondary battery with the sintered nickel electrode result in brittleness of the sintered nickel substrate. Thus, the sintered nickel electrode is susceptible to improvement in the charge/discharge cycle characteristics.
In this connection, proposals have been made in the art as follows. For instance, Japanese Unexamined Patent Publication No.1(1989)-200555 discloses a process aimed at the increase in the conductivity of the active material for improved utilization thereof, the process comprising the steps of laying a cobalt hydroxide layer on a surface of the active material loaded into the porous sintered substrate, and oxidizing the cobalt hydroxide layer by heat treatment in the presence of oxygen and an alkaline solution. Further, Japanese Unexamined Patent Publication No.63(1985)-216268 discloses a process aimed at the suppression of corrosion of the sintered nickel substrate during the loading of the active material and the improvement in the charge/discharge cycle characteristics of the alkaline secondary battery, the process comprising the steps of laying a cobalt hydroxide layer on a surface of a porous sintered nickel substrate, heating the substrate in the presence of oxygen and an alkaline solution, and then loading the nickel hydroxide-based active material into the sintered nickel substrate.
Unfortunately, where the sintered nickel electrode of Japanese Unexamined Patent Publication No.1-200555 is used as the positive electrode of the alkaline secondary battery, the alkaline secondary battery still suffers the occurrence of self discharge due to the oxygen evolution in the sintered nickel electrode when the charged battery is stored at high temperatures of about 50xc2x0 C. over an extended period of time. Thus, the alkaline secondary battery is reduced in capacity.
Where the sintered nickel electrode of Japanese Unexamined Patent Publication No.63-216268(Japanese Examined Patent Publication No.5(1993)-50099) is used as the positive electrode of the alkaline secondary battery, as well, the oxygen evolution occurs in the alkaline secondary battery charged at high temperatures of about 50xc2x0 C. before the positive electrode is charged to full. As a result, the battery is decreased in charge efficiency.
Further, Japanese Unexamined Patent Publication No.48(1973)-50233 has proposed a sintered nickel electrode employing a positive-electrode active material incorporating yttrium hydroxide for improvement in the utilization thereof under high temperature conditions. Alternatively, Japanese unexamined Patent Publication No.5(1993)-28992 discloses an alkaline secondary battery employing a nickel oxide-based active material with a compound, such as yttrium, indium, antimony and the like, added thereto for accomplishing improvement in the utilization of the active material under high temperature conditions.
In those batteries of the above official gazettes, however, the compounds such as of yttrium or the like, are simply added to the active materials and thud, the active materials or the sintered nickel substrates are not sufficiently covered with the compounds such as of yttrium of the like. This detrimentally allows for contact between the electrolyte and the active material and/or the sintered nickel substrate. Hence, there still exists the problem of the oxygen evolution in the nickel electrode under high temperature conditions and of the underutilization of the active material.
In the previous PCT application (PCT/JP99/00720), the present inventors have proposed a nickel electrode for alkaline secondary battery wherein a coating layer is laid on a surface portion of the nickel hydroxide-based active material loaded into the sintered nickel substrate, and is based on a hydroxide of at least one element selected from the group consisting of calcium, strontium, scandium, yttrium, lanthanide and bismuth, as well as a nickel electrode for alkaline secondary battery wherein an intermediate layer is interposed between the sintered nickel substrate and the above active material, and is based on a hydroxide of at least one element selected from the group consisting of calcium, strontium, scandium, yttrium, lanthanide and bismuth.
Where such a nickel electrode for alkaline secondary battery is used as the positive electrode for alkaline secondary battery, the self discharge due to the oxygen evolution in the nickel electrode is suppressed during the long term storage of the charged alkaline secondary battery under high temperature conditions. Thus are provided the alkaline secondary batteries excellent in high temperature storability.
Recently, however, there is an additional desire for a further increased discharge capacity at high current in order to cope with the aforementioned favorable use of the alkaline secondary batteries in the electric power tools.
An object of the invention is to improve the nickel electrode for alkaline secondary battery comprising a porous sintered nickel substrate loaded with a nickel hydroxide-based active material, for suppression of the self discharge of the alkaline secondary battery employing this nickel electrode as the positive electrode and for enhancement of the storability of the battery stored under high temperature conditions.
Another object of the invention is to improve the alkaline secondary battery employing the above nickel electrode as the positive electrode in the high density current charge/discharge characteristics (high-rate characteristics).
According to a first aspect of the invention, a nickel electrode for alkaline secondary battery including a porous sintered nickel substrate loaded with a nickel hydroxide-based active material, the nickel electrode comprises a first coating layer of cobalt compound laid on a surface portion of the active material loaded into the sintered nickel substrate; and a second coating layer laid on the first coating layer and based on a compound of at least one element selected from the group consisting of nickel, magnesium, calcium, barium, strontium, scandium, yttrium, lanthanide and bismuth.
The lanthanide in the second coating layer of the nickel electrode for alkaline secondary battery according to the first aspect hereof may be composed of at least one element selected from the group consisting of lanthanum, cerium, praseodymium, neodymium, samarium, europium and ytterbium.
According to a second aspect of the invention, a nickel electrode for alkaline secondary battery including a porous sintered nickel substrate loaded with a nickel hydroxide-based active material, the nickel electrode comprises a first coating layer of cobalt compound laid on a surface portion of the active material loaded into the sintered nickel substrate, and a second coating layer laid on the first coating layer and based on a complex compound of cobalt and at least one element selected from the group consisting of nickel, magnesium, calcium, barium, strontium, scandium, yttrium, lanthanide and bismuth.
The lanthanide in the second coating layer of the nickel electrode for alkaline secondary battery according to the second aspect hereof may be composed of at least one element selected from the group consisting of lanthanum, cerium, praseodymium, neodymium, samarium, europium and ytterbium.
Where an alkaline secondary battery is fabricated using the nickel electrode of the first or second aspect hereof as the positive electrode therefor, the first and second coating layers on the surface portion of the active material loaded into the porous sintered nickel substrate serve to prevent the electrolyte from coming into contact with the active material and the sintered nickel substrate. Additionally, the cobalt compound in the first coating layer increases conductivity, thus contributing to an increased discharge capacity. On the other hand, because of the compound of nickel, magnesium and/or the like or the complex compound of cobalt and magnesium and/or the like forming the second coating layer, the oxygen evolution is suppressed and hence, the self discharge is prevented during storage of the charged battery under high temperature conditions. This results in the improvement of the high temperature storability.
In the nickel electrode for alkaline secondary battery of the first and second aspects hereof, the second coating layer may preferably employ such a compound of nickel, magnesium and/or the like or such a complex compound of cobalt and magnesium and/or the like as that which is relatively stably present in the alkaline secondary battery. For this reason, the above compound or complex compound is preferably composed of a hydroxide, an oxide or a mixture of these.
If, in the second coating layers of the nickel electrodes of the first and second aspects hereof, the compound of nickel, magnesium and/or the like or the complex compound of cobalt and nickel, magnesium and/or the like contains the above element(s) (including cobalt) in insufficient concentrations, the alkaline secondary battery is not sufficiently improved in the high temperature storability. On the other hand, if the element(s) are contained in excessive concentrations, the alkaline secondary battery cannot attain a sufficient battery capacity because the loading ratio of the active material in the nickel electrode is decreased. Accordingly, the compound or complex compound of the second coating layer preferably contains the above element(s) (including cobalt) in the range of 0.05 to 5 wt % based on the total weight of the active material, and first and second coating layers.
The nickel electrode for alkaline secondary battery of the first aspect hereof may be fabricated by the steps of: dipping the sintered substrate loaded with the nickel hydroxide-based active material in a solution of cobalt salt and then dipping this sintered substrate in an aqueous alkaline solution thereby forming the first coating layer of cobalt compound on the active-material surface portion; and dipping the sintered substrate in a solution containing a salt of at least one element selected from the group consisting of nickel, magnesium, calcium, barium, strontium, scandium, yttrium, lanthanide and bismuth and then dipping the sintered substrate in the aqueous alkaline solution thereby forming the second coating layer on the first coating layer.
The nickel electrode for alkaline secondary battery of the second aspect hereof may be fabricated by the steps of: dipping the sintered substrate loaded with the nickel hydroxide-based active material in a solution of cobalt salt and then dipping this sintered substrate in the aqueous alkaline solution thereby forming the first coating layer of the cobalt compound on the active-material surface portion; and dipping the sintered substrate in a solution containing a cobalt salt and a salt of at least one element selected from the group consisting of nickel, magnesium, calcium, barium, strontium, scandium, yttrium, lanthanide and bismuth and then dipping the sintered substrate in the aqueous alkaline solution thereby forming the second coating layer on the first coating layer.
According to a third aspect of the invention, a nickel electrode for alkaline secondary battery including a porous sintered nickel substrate loaded with a nickel hydroxide-based active material, the nickel electrode comprises a layer laid between a surface portion of the active material loaded into the sintered nickel substrate and/or the sintered nickel substrate, and the active material, the layer based on a complex compound of nickel and at least on element selected from the group consisting of cobalt, calcium, strontium, scandium, yttrium, lanthanide, bismuth, magnesium and barium.
The lanthanide in the above layer may be composed of at least one element selected from the group consisting of lanthanum, cerium, praseodymium, neodymium, europium and ytterbium.
Where an alkaline secondary battery is fabricated using the nickel electrode of the third aspect hereof as the positive electrode therefor, the above layer between the surface portion of the active material loaded into the porous sintered nickel substrate and/or the sintered nickel substrate, and the active material prevents the electrolyte from coming into contact with the active material and sintered nickel substrate.
In the nickel electrode for alkaline secondary battery of the third aspect hereof, the layer between the active-material surface portion and/or the sintered nickel substrate, and the active material contains the complex compound of nickel and the aforesaid element(s) and therefore, the nickel compound of this layer promotes the charge/discharge reactions thereby improving the alkaline secondary battery in the high-current discharge capacity. In addition, the compound of the aforesaid element(s), such as cobalt and calcium and the like, suppresses the oxygen evolution during storage of the charged battery under high temperature conditions and hence, the self discharge is prevented. This results in the enhanced high temperature storability.
In the nickel electrode for alkaline secondary battery of the third aspect hereof, the above layer may preferably employ such a complex compound as that which is relatively stably present in the alkaline secondary battery. For this reason, the above complex compound is preferably composed of a hydroxide, an oxide or a mixture of these.
In the layer based on the complex compound of nickel and the aforesaid element(s) and laid between the surface portion of the active material loaded into the sintered nickel substrate and/or the sintered nickel substrate, and the active material, if the aforesaid complex compound is contained in insufficient concentrations, inabilities to adequately suppress the reaction between the electrolyte and the active material and the like and to adequately improve the high temperature storability of the alkaline secondary battery result. On the other hand, if the complex compound is contained in excessive concentrations, the alkaline secondary battery cannot attain a sufficient capacity because the loading ratio of the active material in the nickel electrode is decreased. Accordingly, a mixing ratio of the complex compound in the aforesaid layer is preferably in the range of 0.5 to 5 wt % based on the total weight of all the loaded materials which include the nickel hydroxide-based active material. Further, the above layer preferably contains the compound of the element(s) selected from the group consisting of cobalt, calcium, strontium, scandium, yttrium, lanthanide, bismuth, magnesium and barium in a total weight percentage of 0.3 to 3 wt % based on the total weight of all the loaded materials which include the nickel hydroxide-based active material loaded into the sintered nickel substrate.
According to a fourth aspect of the invention, a nickel electrode for alkaline secondary battery including a porous sintered nickel electrode loaded with a nickel hydroxide-based active material, the nickel electrode comprises a first coating layer which is laid on a surface portion of the active material loaded into the sintered nickel substrate and is based on a compound of at least one element selected from the group consisting of magnesium, calcium, barium, strontium, scandium, yttrium, lanthanide and bismuth, and a second coating layer of cobalt compound laid on the first coating layer.
According to a fifth aspect of the invention, a nickel electrode for alkaline secondary battery including a porous sintered nickel substrate loaded with a nickel hydroxide-based active material, the nickel electrode comprises a first coating layer which is laid on a surface portion of the active material loaded into the sintered nickel substrate and is based on a complex compound of cobalt and at least one element selected from the group consisting of magnesium, calcium, barium, strontium, scandium, yttrium, lanthanide and bismuth, and a second coating layer of cobalt compound laid on the first coating layer.
The lanthanide in the first coating layer of the nickel electrodes for alkaline secondary battery according to the fourth and fifth aspects hereof may be composed of at least one element selected from the group consisting of lanthanum, cerium, praseodymium, neodymium, europium and ytterbium.
Where an alkaline secondary battery is fabricated using the nickel electrode of the fourth or fifth aspect hereof as the positive electrode therefor, the first and second coating layers laid on the surface portion of the active material loaded into the porous sintered nickel substrate prevent the electrolyte from coming into contact with the active material and sintered nickel substrate. Additionally, because of the compound of magnesium, calcium and/or the like or the complex compound of cobalt and magnesium, calcium and/or the like in the first coating layer, the oxygen evolution is suppressed and hence, the self discharge is prevented during storage of the charged battery under high temperature conditions. This results in the improvement in the high temperature storability. At the same time, the cobalt compound in the second coating layer increases conductivity, allowing for discharge at high voltage. Particularly, as suggested by the nickel electrode of the fifth aspect hereof, the first coating layer, which is based on the complex compound of cobalt and magnesium, calcium and/or the like, is further improved in conductivity, thus contributing to an even greater high temperature storability and allowing for discharge at an even higher voltage.
In the nickel electrodes for alkaline secondary battery of the fourth and fifth aspects hereof, the first coating layer may preferably employ such a compound of magnesium, calcium and the like or such a complex compound of cobalt and magnesium, calcium and/or the like as that which is relatively stably present in the alkaline secondary battery. For this reason, the above compound or complex compound is preferably composed of a hydroxide, an oxide or a mixture of these.
In the nickel electrodes of the fourth and fifth aspects hereof, if the first coating layer contains the compound of magnesium, calcium or the like, or the complex compound of cobalt and magnesium, calcium and/or the like in insufficient concentrations, inabilities to adequately suppress the reaction between the electrolyte and the active material and to sufficiently improve the high temperature storability result. On the other hand, if the compound or complex compound is contained in excessive concentrations, the battery cannot attain a sufficient battery capacity because the loading ratio of the active material in the nickel electrode is decreased. Accordingly, a mixing ratio of the compound or complex compound in the first coating layer is preferably in the range of 0.5 to 5 wt % based on the total weight of the active material, and first and second coating layers.
In the nickel electrodes of the fourth and fifth aspects hereof, if the second coating layer contains the cobalt compound in insufficient concentrations, the nickel electrode for alkaline secondary battery is not sufficiently improved in conductivity and an inability to discharge at high voltage results. On the other hand, if the compound is contained in excessive concentrations, the battery cannot attain a sufficient battery capacity because the loading ratio of the active material in the nickel electrode is decreased. Accordingly, a mixing ratio of the cobalt compound in the second coating layer is preferably in the range of 0.5 to 5 wt % based on the total weight of the active material, and first and second coating layers.
According to a sixth aspect of the invention, a nickel electrode for alkaline secondary battery including a porous sintered nickel substrate loaded with a nickel hydroxide-based active material, the nickel electrode comprises a layer which is laid between a surface portion of the active material on the porous sintered nickel substrate and/or the sintered nickel substrate, and the active material, and which is based on a complex compound of at least one element selected from the group consisting of cobalt and nickel, and at least one element selected from the group consisting of manganese, aluminum, iron, copper and silver.
Where an alkaline secondary battery is fabricated using the nickel electrode of the sixth aspect hereof as the positive electrode therefor, the aforesaid layer, which is laid between the surface portion of the active material loaded into the porous sintered nickel substrate and/or the sintered nickel substrate, and the active material, prevents the electrolyte from coming into contact with the active material and sintered nickel substrate.
In the nickel electrode for alkaline secondary battery according to the sixth aspect hereof, the layer between the surface portion of the active material and/or the sintered nickel substrate and the active material employs the complex compound of at least one element selected from the group of cobalt and nickel and at least one element selected from the group of manganese, aluminum, iron, copper and silver, such that the self discharge due to the oxygen evolution is prevented during storage of the charged electrode under high temperature conditions. Thus, the alkaline secondary battery is improved in the high temperature storability. In addition, the electrode with this complex compound provides smoother charge/discharge reactions than the conventional sintered nickel electrode formed with a manganese hydroxide layer over its surface, thus allowing for the discharge at high voltage. Furthermore, the complex compound also suppresses the expansion of the nickel electrode for alkaline secondary battery thereby improving the charge/discharge cycle characteristics of the battery.
In the nickel electrode for alkaline secondary battery of the sixth aspect hereof, the above layer may preferably employ such a complex compound as that which is relatively stably present in the alkaline secondary battery. For this reason, the above complex compound is preferably composed of a hydroxide, an oxide or a mixture of these.
In the layer which is laid between the surface portion of the active material loaded into the sintered nickel substrate and/or the sintered nickel substrate, and the active material and which is based on the above complex compound, if the layer contains the complex compound in insufficient concentrations, inabilities to adequately suppress the reaction between the electrolyte and the active material and the like and to sufficiently improve the alkaline secondary battery in the high temperature storability result. If, on the other hand, the layer contains the complex compound in excessive concentrations, the battery cannot attain a sufficient battery capacity because the loading ratio of the active material in the nickel electrode for alkaline secondary battery is decreased. Besides, the alkaline secondary battery is lowered in the charge/discharge cycle characteristics because of decreased discharge voltage. Accordingly, a mixing ratio of the complex compound in the above layer is preferably in the range of 0.5 to 5 wt % based on the total weight of all the loaded materials which include the nickel hydroxide-based active material. Further, the above layer preferably contains the compound of the element(s) selected from the group of manganese, aluminum, iron, copper and silver in a total weight percentage of 0.3 to 3 wt % based on the total weight of all the loaded materials which include the nickel hydroxide-based active material loaded into the sintered nickel substrate.
According to a seventh aspect of the invention, a nickel electrode for alkaline secondary battery including a porous sintered nickel substrate loaded with a nickel hydroxide-based active material, the nickel electrode comprises a coating layer which is laid on a surface portion of the active material loaded into the sintered nickel substrate and which contains a complex compound of at least one element selected from the group consisting of yttrium and ytterbium and at least one element selected from the group consisting of manganese, aluminum, iron, copper and silver.
According to an eighth aspect of the invention, a nickel electrode for alkaline secondary battery including a porous sintered nickel substrate loaded with a nickel hydroxide-based active material, the nickel electrode comprises a coating layer which is laid on a surface portion of the active material loaded into the sintered nickel substrate and which contains a complex compound of at least one element selected from the group consisting of yttrium and ytterbium, at least one element selected from the group consisting of manganese, aluminum, iron, copper and silver, and at least one element selected from the group consisting of cobalt and nickel.
Where an alkaline secondary battery is fabricated using the nickel electrode for alkaline secondary battery of the seventh or eight aspect hereof as the positive electrode therefor, the above coating layer on the surface portion of the active material loaded into the porous sintered nickel substrate prevents the electrolyte from coming into contact with the active material and sintered nickel substrate, thereby suppressing the self discharge.
In the nickel electrodes for alkaline secondary battery according to the seventh and eighth aspects hereof, the coating layers employ the aforesaid complex compound, yttrium and/or ytterbium of which is more effective to suppress the oxygen evolution and hence the self discharge in the charged electrode stored under high temperature conditions, as compared with the conventional sintered nickel electrode formed with the manganese hydroxide layer over its surface. Thus, the alkaline secondary battery is improved in the high temperature storability and the charge efficiency at high temperatures. The complex compounds also provide smooth charge/discharge reactions, allowing for discharge at high voltage. In addition, the expansion of the nickel electrode for alkaline secondary battery is prevented and thus, the alkaline secondary battery is improved in the charge/discharge cycle characteristics.
In the nickel electrode for alkaline secondary battery of the eight aspect hereof, the complex compound of the coating layer further contains at least one element selected from the group of cobalt and nickel so that the coating layer is improved in the conductivity and the battery reactions. This allows for high voltage discharge and contributes to the improvement of the charge/discharge cycle characteristics.
In the nickel electrodes for alkaline secondary battery of the seventh and eighth aspects hereof, the above coating layers may preferably employ such a complex compound as that which is relatively stably present in the alkaline secondary battery. For this reason, the above complex compound is preferably composed of a hydroxide, an oxide or a mixture of these.
In the nickel electrodes for alkaline secondary battery according to the seventh and eighth aspects hereof, if the coating layers contain the above complex compounds in insufficient concentrations, the coating layers are incapable of adequately suppressing the reactions between the electrolyte and the active material and of sufficiently improving the alkaline secondary batteries in the high temperature storability. If, on the other hand, the coating layers contain the complex compounds in excessive concentrations, the battery cannot attain a sufficient battery capacity because the loading ratio of the active material in the nickel electrode for alkaline secondary battery is decreased. In addition, the alkaline secondary battery is lowered in the discharge voltage and suffers the expansion of the nickel electrode thereof which results from the increase in an undischarged portion of the electrode. This leads to lowered charge/discharge cycle characteristics of the battery. Accordingly, a mixing ratio of the complex compound in the coating layer is preferably in the range of 0.5 to 5 wt % based on the total weight of the active material and coating layer.
If the above complex compound contains the compound of the element(s) selected from the group of manganese, aluminum, iron, copper and silver in insufficient concentrations, the alkaline secondary battery is lowered in the high temperature storability and cycle characteristics. If, on the other hand, the complex compound contains such a compound in excessive concentrations, the alkaline secondary battery is lowered in the working voltage and the charge efficiency at high temperatures. Hence, the complex compound preferably contains the compound of the element(s) selected from the group of manganese, aluminum, iron, copper and silver in a total weight percentage of 0.3 to 3 wt % and more preferably 2 to 3 wt % based on the total weight of the active material and coating layer.
If the complex compound contains the compound of the element(s) selected from the group of yttrium and ytterbium in insufficient concentrations, the alkaline secondary battery is lowered in the working voltage and the charge efficiency at high temperatures. If, on the other hand, the complex compound contains such a compound in excessive concentrations, the alkaline secondary battery is lowered in the high temperature storability and cycle characteristics. Hence, the complex compound preferably contains the compound of the element(s) selected from the group of yttrium and ytterbium in a total weight percentage of 0.3 to 3 wt % and more preferably 2 to 3 wt % based on the total weight of the active material and coating layer.
Incidentally, it is preferred that zinc, cadmium, magnesium, cobalt, manganese or the like is incorporated into the nickel hydroxide-based active material as solid solution for the prevention of the expansion of the nickel electrode during the charge/discharge processes of the alkaline secondary battery employing any of the nickel electrodes of the first to eighth aspects hereof.